When fabricating optical components such as lenses, a coating is often formed on a surface of the component, to provide desired optical or physical properties. For example, the coating may provide an anti-reflective (AR) characteristic, a filtering characteristic, physical protection for the component, some other characteristic, or a combination of two or more characteristics. These coatings often include multiple layers of different materials that collectively provide the desired characteristic(s).
The layers in a coating should ideally have a uniform thickness, and the optical and mechanical characteristics of the layer should ideally be uniform throughout the layer. But as a practical matter, this is not always the case. For example, where a coating is formed on a relatively highly curved surface, it is common for a given layer within the coating to have a peripheral region that is as much as 30% to 50% thinner than a central region of that layer, or even more than 50% thinner. Further, different layers in the same coating often have different degrees of variation in thickness. For example, one layer may be 30% thinner in a peripheral region than in a central region, while another layer may be 50% thinner in the peripheral region that in the central region. Thus, even assuming that the layers of a coating all have the proper thicknesses in the central region, the thicknesses in the peripheral region will typically not be correct and, moreover, the ratios of thickness in the peripheral region will not be correct. As a result, the coating will provide the desired optical and mechanical characteristics in its central region, but may fail to provide these desired characteristics in its peripheral region, or may at least exhibit a degradation of these characteristics in the peripheral region.
One existing approach for analyzing a coating is to use a spectrophotometer to measure transmissivity and/or reflectance of the coating, at different locations on the coating. While existing approaches of this type have been generally adequate for their intended purposes, they have not been satisfactory in all respects.